It is quite common to consider the terms ambiguous and confusing as synonyms. Confusing information brings together various data with similar meanings. In ambiguous information, on the other hand, several meanings are assigned to a single word. Excessive information also generates ambiguity; therefore, a concise, clear language is demanded. The term adverse event (AE) is defined as any inconvenient medical occurrence suffered by a subject during a clinical investigation research. Confusion and ambiguity in the use of words may generate relevant consequences in the appraisal of AEs. The objective of this present theoretical study is to harmonize the vocabulary applied in the characterization of risks and in the communication of AEs in clinical research processes. AEs may be classified according to their predictability, frequency, gravity, causality, and severity. Regulatory documents usually address AEs in their severity and causality aspects. Vocabulary conformity in the communication of AEs is an essential step towards avoiding inaccurate use of words with confused or ambiguous meanings.